Communications between aircraft are usually referred to as “intra-flight” communications. The presence of a direct path between a transmitter positioned on a first aircraft and a receiver positioned on a second aircraft is the norm. Indirect paths may also be received by said aircraft. These indirect paths may be the consequence of reflection or refraction of the transmitted signal by the structure of said aircraft. They may also be the consequence of reflection of the signal from the ground or the sea, said paths being in this case usually referred to as ground echoes and sea echoes. These reflections create interference; their power may be high, in particular where sea echoes are concerned, notably when the swell is low.
The form of interference resulting from these echoes is difficult to cancel out or to equalize because an echo may be distant and time-shifted by a duration much greater than a modulation symbol, giving rise to harmful intersymbol interference.
Classic equalizers, whether they are used for terrestrial communications or for communications between aircraft, equalize the channel around a small range of delays around the signal. For example, in GSM radio communication, equalizers with five coefficients may be used to process paths subject to a delay not exceeding 20 μs. This type of equalizer is notably described by T. Lucidarme in the book entitled Principes de radiocommunication de troisième génération, Vuibert, 2002.
In the context of communication between aircraft and/or spacecraft, reflections caused by ground echoes and sea echoes imply long delays and are usually not processed because of the complexity this would add to the receivers.